As described in JP-A-10-56413, a compression ignition internal combustion engine adopts a combustion system of compressing and self-igniting a uniformly premixed fuel-air mixture. The compression ignition internal combustion engine can operate in an ultra-lean area (air-fuel ratio of 80 or more) that cannot be operated by a conventional gasoline engine or diesel engine, and decreases flame temperatures and realizes ignition combustion with a uniform fuel-air mixture, and therefore it is an engine that allows drastic reduction of both NOx and soot.
Generally, when a fuel-air premixture is compressed and reached at a certain temperature, a reaction called “low temperature oxidation reaction” whose initial reaction is dehydrogenation of hydrocarbon as a fuel, starts. When this reaction progresses, an elementary reaction called “blue flame” takes place, which leads to self-ignition. Since this ignition takes place at multiple points in the fuel-air mixture simultaneously, the combustion period for the combustion chamber as a whole is by far shorter than combustion by spark ignition of a conventional gasoline engine or a combustion period of injection combustion of a diesel engine. For this reason, this results in a reduction of flame temperature and suppression of NOx generation which is dependent on a duration thereof, which constitutes a factor of realizing low NOx in the compression ignition internal combustion engine.
However, the conventional compression ignition internal combustion engine has a problem that its output torque range is limited to a very narrow range and an engine operation with compression ignition is only realized within quite a limited range of low-load and low-speed rotation. The reason is that the temperature for a fuel-air premixture using hydrocarbon as a fuel to reach self-ignition is said to be 900K or higher and a current gasoline engine whose compression ratio is set to about 10 to 13 is known to have almost no operating area where self-ignition can take place. Furthermore, the compression ratio can be set as high as that of a diesel engine (16 to 22) and there can be an engine operating area by self-ignition of a fuel-air premixture, but since it is difficult for the conventional engine to control self-ignition timings of the mixture, a combustion period is short and compression self-ignition of the premixture is strongly affected by an air-fuel ratio, etc., its output torque range is limited to a very narrow range, causing a problem that an engine operation by compression ignition can be realized only in quite a limited area of low-load and low-speed rotation.
In contrast, as described, for example, in JP-A-11-280507, an engine is known which flows backward a high-temperature burnt gas (internal EGR) generated in a previous cycle to a combustion chamber by operating a mechanism that makes variable valve timings of intake/exhaust valves, keeps the inside of the combustion chamber at a high temperature through the internal EGR in a low-load area and realizes an operating area by the self-ignition combustion through control over the amount of the internal EGR and real compression ratio and operates by the spark ignition combustion in a high-load area.